Narbacular Drop is a student project/indie game. It's most well known for being the predecessor to the game Portal. When it originally released the game had a surprisingly active modding community during its first year. The developers used Hammer to build the levels, despite the game running on an entirely custom engine.
The level editor pack and downloads of all the levels used to be available on the developer's website. However the developer's website has long fallen into disrepair and the links are now broken. I have taken the liberty of reuploading all of the levels and utilities to Moddb so they never go lost again. I really wanted to preserve this lost modding history as the vast majority of people I've spoken to had no idea the game had a modding community.